CMOS imagers are known in the art. A top-down view of a semiconductor wafer fragment of an exemplary CMOS sensor pixel four-transistor (4T) cell 10 is illustrated in FIG. 1. As it will be described below, the CMOS sensor pixel cell 10 includes a photo-generated charge accumulating area 21 in an underlying portion of the substrate. This area 21 is formed as a pinned diode 11 (FIG. 2). The pinned photodiode is termed “pinned” because the potential in the photodiode is pinned to a constant value when the photodiode is fully depleted. It should be understood, however, that the CMOS sensor pixel cell 10 may include a photogate or other image to charge converting device, in lieu of a pinned photodiode, as the initial accumulating area 21 for photo-generated charge.
The CMOS image sensor 10 of FIG. 1 has a transfer gate 30 for transferring photoelectric charges generated in the charge accumulating region 21 to a floating diffusion region (sensing node) 25. The floating diffusion region 25 is further connected to a gate 50 of a source follower transistor. The source follower transistor provides an output signal to a row select access transistor having gate 60 for selectively gating the output signal to terminal 32. A reset transistor having gate 40 resets the floating diffusion region 25 from a supply voltage applied at a source/drain region between gates 40 and 50 to a specified charge level before each charge transfer from the charge accumulating region 21.
A cross-sectional view of the exemplary CMOS image sensor 10 of FIG. 1 taken along line 2-2′ is illustrated in FIG. 2. The charge accumulating region 21 is formed as a pinned photodiode 11 which has a photosensitive or p-n-p junction region formed by a p-type layer 24, an n-type region 26 and the p-type substrate 20. The pinned photodiode 11 includes two p-type regions 20, 24 so that the n-type photodiode region 26 is fully depleted at a pinning voltage. Impurity doped source/drain regions 22 (FIG. 1), preferably having n-type conductivity, are provided on either side of the transistor gates 40, 50, 60. The floating diffusion region 25 adjacent the transfer gate 30 is also preferably n-type.
Generally, in CMOS image sensors such as the CMOS image sensor cell 10 of FIGS. 1-2, incident light causes electrons to collect in region 26. A maximum output signal, which is produced by the source follower transistor having gate 50, is proportional to the number of electrons to be extracted from the region 26. The maximum output signal increases with increased electron capacitance or acceptability of the region 26 to acquire electrons. The electron capacity of pinned photodiodes typically depends on the doping level of the image sensor and the dopants implanted into the active layer.
FIG. 2 also illustrates trench isolation regions 15 formed in a substrate layer 20 in which the charge accumulating region 21 is formed. The substrate layer 20 may be an epitaxial layer provided over a silicon base layer. The trench isolation regions 15 are typically formed using a conventional STI process or by using a Local Oxidation of Silicon (LOCOS) process. Trench isolation regions 15 provide a physical barrier between adjacent pixels and help isolate pixels optically and electrically from one another. For example, as shown in FIG. 2, trench isolation region 15 provides surface electrical isolation of the pinned photodiode 11 of pixel region A from the photosensitive elements of adjacent pixel region B.
Another problem associated with the formation of the above-described trench isolation regions 15 is that, when ions are implanted in the substrate close to bottom 17 and edges or sidewalls 16 (FIG. 2) of the trench, current leakage can occur at the junction between the active device regions and the trench. In addition, the dominant crystallographic planes along the bottom 17 and sidewalls 16 of the trench isolation regions 15 have a higher silicon density than the adjacent silicon substrate and, therefore, create a high density of trap sites along the trench bottom 17 and sidewalls 16. These trap sites are normally uncharged but become charged when electrons and holes become trapped in the trap sites. As a result of these trap sites formed along the bottom 17 and sidewalls 16 of the trench isolation regions 15, current generation near and along the trench bottom 17 and sidewalls 16 can be very high and can reduce the photogenerated charge otherwise available from light captured by the photodevice. Current generated from trap sites inside or near the photodiode depletion region also causes undesired dark current.
Minimizing dark current in the photodiode is important in CMOS image sensor fabrication. Dark current is generally attributed to leakage in the charge collection region 21 of the pinned photodiode 11, which is strongly dependent on the doping implantation conditions of the CMOS image sensor. In addition and as explained above, defects and trap sites inside or near the photodiode depletion region strongly influence the magnitude of dark current generated. In sum, dark current is a result of current generated from trap sites inside or near the photodiode depletion region, surface leakage at silicon/surface interface; band-to-band tunneling induced carrier generation as a result of high fields in the depletion region; junction leakage coming from the lateral sidewall of the photodiode; and leakage from isolation corners, for example, stress induced and trap assisted tunneling.
CMOS imagers also typically suffer from poor signal to noise ratios and poor dynamic range as a result of the inability to fully collect and store the electric charge collected in the region 26. Since the size of the pixel electrical signal is very small due to the collection of photons in the photo array, the signal to noise ratio and dynamic range of the pixel should be as high as possible.
There is needed, therefore, an improved CMOS imager that exhibits reduced pixel spacing between adjacent pixel sensor cells, reduced cross-talk and blooming, as well as reduced dark current and increased photodiode capacitance. There is also needed an isolation region for a CMOS imager and that (i) has minimum width to increase the pixel fill factor (pixel density) while providing electrical and physical separation between photosensitive elements of adjacent pixels of the CMOS imager; (ii) prevents current generation or current leakage; and (iii) acts as a reflective barrier to electrons generated by light absorption in a photodiode back to the charge collection region of the photodiode of a pixel sensor cell.
A method of fabricating active pixel photosensors exhibiting these improvements is also needed, as well as an isolation technique that eliminates dark current and reduces cross-talk between adjacent pixel sensor cells of a CMOS imager.